The present invention relates to digital optical switches. In particular, it relates to tightly curved digital optical switches.
The shift to optical fiber as the preferred transmission medium in telecommunications has created the need for practical optical switches to perform both the routing and distribution functions of the optical network. A type of optical switch is the waveguide spatial switch that selectable transfers an incoming optical signal to a specific one of a plurality of output waveguides. The spatial switch is controlled by an externally applied electrical control signal without regard to the particular spectral or informational content of that signal. This type of optical switch generally functions in one of three modes: (1) as a direct coupler switch, (2) as a gain switch, or (3) as an index switch. A direct coupler switch splits single polarized monochromic light and subsequently recombines it constructively in the desired channel and destructively in the non-desired channel. A gain switch splits the light and subsequently amplifies the light in the desired channel (the amplification compensates for the loss from splitting the light) and absorbs it in the non-desired channel. An optical index switch focuses, or reflects, the optical energy into the desired output channel.
A number of factors, however, limit the performance of these different switches. For example, direct coupler switches are critically dependent on the length of the active interference region for complete switching. They must be fabricated for operation at a particular wavelength and polarization, which eliminates use in a multi-wavelength network or a network in which the polarization varies. Gain switches suffer from the high power requirements. Additionally, gain switches require complex fast optical amplifiers in the output channels, which must be turned on for amplification or off for absorption. Finally, optical switches that depend upon mechanical or thermal mechanisms for switching are too slow for general use in a telecommunication network.
This leaves the optical index switch as the switch of choice for most telecommunication networks. Optical switches can be fabricated to reflect or to focus light by changes in the index of refraction, where the index change is accomplished by either injecting or depleting electrical carriers in the switching region. The depletion mode of operation suffers from polarization sensitivity because the depletion region's large electrical field couples to the material's electro-optic coefficients; however, for polarization insensitive operation, the optical index switch can be operated by injecting electrical carriers.
Conventional optical index switches require electrodes on each output branch to effect index changes so that a propagation tendency in the higher index branch will result due to asymmetric adiabatic modal evolution. However, these index switches require stringent fabrication procedure and tolerances to minimize any coupling between the electrodes, particularly in the interaction region at the wave guide junction. Moreover, index switches require continuous electrical power to remain in a particular switching state.
Solutions to the above problems were provided by U.S. Pat. No. 5,490,226 for ZERO HOLDING POWER DIGITAL OPTICAL SWITCHES and U.S. Pat. No. 5,537,497 for OPTIMIZED ELECTRODE GEOMETRIES FOR DIGITAL OPTICAL SWITCHES, both issued to William H. Nelson, the relevant disclosure of which is set out below. These switches, however, required a large chip in order to achieve the required separation between the terminations that would allow coupling to individual fibers. In order to reduce the length of the switch, current technology requires additional masking and regrowth, or fabrication, to place tightly bound curved waveguides in the switch. The additional masking and regrowth increases the cost and significantly reduces the yield of the optical switch. Therefore, it would be desirable to provide a small chip without the additional masking and regrowth, and the corresponding reduction in yield.